1. Field of the Invention
The invention relates to a process for producing catalysts on stable, high-purity shaped bodies comprising pyrogenic metal oxides without additional binders and their use in the gas-phase oxidation of olefins.
2. Description of the Related Art
Pyrogenic metal oxides have an extremely small particle size, high specific surface areas, defined, spherical primary particles having a defined surface chemistry and do not have any internal surface (pores). Furthermore, they have a very high chemical purity.
Due to the properties outlined above, pyrogenic silicon dioxides, for example, are attracting increasing interest as, supports for catalysts (D. Koth, H. Ferch, Chem. Ing. Techn. 52, 628 (1980)).
However, owing to the particularly small particle size of pyrogenic metal oxides, the production of shaped bodies as are used, for example, as catalyst or catalyst support from these pyrogenic metal oxides is difficult. Shaped bodies comprising metal oxide powders are generally produced by pressing or extrusion using binders and lubricants in order to obtain stable shaped bodies. The binders and lubricants are inorganic and organic additives.
Inorganic additives, e.g. magnesium stearate, remain in the shaped bodies produced in the form of inorganic compounds, e.g. magnesium oxide. Organic additives, too, can result in impurities such as carbon in the production process for the shaped bodies. The desired very high purity of the pyrogenic metal oxides used, e.g. pyrogenic SiO2, is therefore lost in the shaped bodies produced.
Apart from the high purity and the high surface area, a very low bulk density of the catalysts is a further desired property. This can, firstly, have a favourable effect on material transport in the future catalyzed reaction and, secondly, a low mass of support material is required in order to fill a particular reactor volume. This improves the cost ratio of support material to reactor volume and makes the process more economical.
Low bulk densities can be achieved, for example, by means of catalyst shapes which have at least one passage through them, for example rings.
Ring-shaped bodies having a very low wall thickness are particularly useful. However, low wall thicknesses lead to shaped bodies whose mechanical strengths are no longer sufficient for catalyst preparation and/or filling of the reactor and which are therefore unsuitable as catalyst support materials.
As catalytically active components, it is possible to use, inter alia, palladium and/or its compounds and alkali metal compounds, and additionally gold and/or its compounds (Pd/alkali metal/Au system) or cadmium and/or its compounds (Pd/alkali metal/Cd system) or barium and/or its compounds (Pd/alkali metal/Ba system) or palladium, alkali metal compounds and mixtures of gold and/or cadmium and/or barium.
The prior art describes many possible ways of producing catalysts on shaped bodies comprising metal oxides. In these, binders or other strengthening steps are necessary to achieve later strength and the surface areas of the catalyst supports obtained require high bulk densities of the catalysts.
EP 72390 describes the production of pressed bodies from a mixture of pyrogenic metal oxides, water, silica sol and a pressing aid. A polyfunctional alcohol (e.g. glycerol) is claimed as an auxiliary.
EP 327722 discloses mixing pyrogenic silicon dioxide together with kaolin and/or graphite, sugar, starch, urea, wax into water. The pressed bodies can be produced using punch presses, excentric presses, extruders, rotary presses or compactors. An analogous procedure is employed in EP 327815, but a pyrogenic mixed oxide of silicon dioxide/aluminium oxide is used in place of pyrogenic silicon dioxide.
EP 807615 describes a process for producing pressed bodies comprising pyrogenic silicon dioxide, methylcellulose, microwax and polyethylene glycol and water. The pressed bodies usually comprise from 50 to 90% by weight of silicon dioxide, from 0.1 to 20% by weight of methylcellulose and from 0.1 to 15% by weight of microwax. and from 0.1 to 15% by weight of polyethylene glycol.
According to DE4142898, it is possible to produce stable shaped bodies from pyrogenic silica and aqueous-alcoholic ammonia solution. In contrast, a purely aqueous ammonia solution does not give a successful result. The high proportion of aqueous-alcoholic ammonia solution makes the mixture to be shaped strongly alkaline. The use of alcohol brings with it the risk of carbon contamination in the resulting catalyst support. According to DE4142902, stable shaped bodies can be obtained from pyrogenic silica and ammonia solution or from pyrogenic silica and silica sol containing alkali metal only when the shaped bodies are subjected to a hydrothermal treatment. In the case of the additional ammonia, the mixture once again becomes very alkaline. It is known that this excess of base (pH >10) leads to partial dissolution of SiO2.
U.S. Pat. No. 4,048,096 describes the production of surface-impregnated Pd/Au catalysts in which the noble metals occur in a layer thickness of less than or equal to 0.5 mm from the outer surface of the support bodies. Alkali metal silicates are used as base for converting the soluble noble metal compounds into the respective noble metal hydroxides and a pH of 6.5-9.5 is set during this step for a time of 12-24 hours. Supports having surface areas of 10-800 square meters per gram are used as support materials for the VAM catalysts based thereon.
EP 807615 describes a process for producing pressed bodies comprising pyrogenic silicon dioxide, in which silicon dioxide is homogenized with methylcellulose, microwax and polyethylene glycol and an addition of water. After mixing, drying is carried out at from 80 to 150° C. The powder, which may-have been comminuted beforehand, is shaped to produce pressed bodies which are heat treated at temperatures of from 400 to 1200° C. for a period of from 0.5 to 8 hours. The mixtures before pressing the masses usually comprise from 50 to 90% by weight of silicon dioxide, from 0.1 to 20% by weight of methylcellulose and from 0.1 to 15% by weight of microwax and from 0.1 to 15% by weight of polyethylene glycol. The pressed bodies described in the examples have BET surface areas of from 120 to 210 m2/g at pore volumes of from 0.71 to 0.97 ml/g. The pressed bodies having an external diameter of from 0.8 to 20 mm and a BET surface area of from 30 to 400 m2/g claimed in the patent have a pore volume of from 0.5 to 1.3 ml/g. The shaped bodies having a bulk density of from 350 to 750 g/l comprise at least 99.8% by weight of silicon dioxide (other constituents <0.2% by weight) and achieve mechanical strengths of from 10 to 250 newton at an attrition of <5% by weight. Catalysts for preparing vinyl acetate monomer which are based on these support materials and contain palladium, gold and alkali metal acetate are claimed.
EP 997192 B1 describes supported palladium catalysts (Pd/Au/alkali metal, Pd/Cd/alkali metal or Pd/Ba/alkali metal systems) which are based on shaped bodies comprising pyrogenic mixed oxides. The shaped bodies on which the catalyst is based have an external diameter of from 0.8 to 25 mm, a BET surface area of from 5 to 400 m2/g and a pore volume of from 0.2 to 1.8 ml/g and are composed of at least two oxides from the group consisting of SiO2, Al2O3, TiO2 and ZrO2 in any order, but with the exception of SiO2/Al2O3 mixed oxides, with other constituents being <1% by weight. In addition, the support materials described have compressive strengths of from 5 to 350 newton and bulk densities of from 250 to 1500 g/l. Catalysts which are based on these support materials and contain palladium, gold and alkali metal compounds or palladium, cadmium and alkali metal compounds or palladium, barium and alkali metal compounds are claimed. The alkali metal compound is potassium acetate in a preferred embodiment. The catalyst is used for preparing unsaturated esters, e.g. vinyl acetate monomer, in the gas phase.
EP 464633 describes a catalyst which has at least one passage having an internal diameter of at least one millimeter going through it and contains palladium and/or compounds thereof in an amount of from 1 to 20 gram/liter and, if appropriate, gold and/or compounds thereof in an amount of from 0.1 to 10 gram/liter. At least 95% of the palladium, gold and/or compounds thereof are present in a region from the surface to up to 0.5 mm below the surface of the support (surface-impregnated or coated catalyst). The catalyst is used for preparing unsaturated esters (e.g. vinyl acetate) by reaction of an olefin (e.g. ethene) with an organic carboxylic acid (e.g. acetic acid) and oxygen in the gas phase. In the examples, hydrazine hydrate is used as reducing agent.
The documents of the prior art show that the production of stable shaped bodies has hitherto not been possible without inorganic or organic additives such as extrusion aids, pore formers, sols or additional strengthening steps. Furthermore, the catalysts from the prior art display unfavourable bulk densities and low activities.